1. Field of the Invention
The present invention relates to a fixing device that fixes a toner image on a recording material by application of heat and pressure. In addition, the present invention also relates to an image forming apparatus, such as a copier, a printer, or a multifunctional apparatus combining these functions, including the fixing device.
2. Description of the Background
An electrophotographic image forming apparatus generally includes a fixing device that fixes a toner image on a recording material by application of heat and pressure. Toner that composes the toner image is generally comprised of a resin fusible by heat.
FIG. 1 schematically illustrates a related-art fixing device including two rollers. A heating roller 101 and a pressing roller 103 form a nip therebetween. The heating roller 101 is driven to rotate clockwise in FIG. 1, and the pressing roller 103 rotates along rotation of the heating roller 101. The heating roller 101 incorporates a heat source 102 that heats the surface of the heating roller 101 above the softening point of toner in use.
An unfixed toner image formed on a recording material is fed to the nip between the heating roller 101 and the pressing roller 103, and receives heat and pressure from the heating roller 101 and the pressing roller 103. The toner image is softened and melted in the nip, and then cooled after passing through the nip, resulting in fixation of the toner image on the recording material.
However, the toner image thus fixed on the recording material by a fixing device having the configuration described above has a problem of low gloss. The reason for the low gloss is considered as follows.
At the exit of the nip between the heating roller 101 and the pressing roller 103, the sufficiently-melted toner image is contacting the smooth surface of the heating roller 101. Therefore, the melted toner image is expressing substantially the same gloss as the smooth surface of the heating roller 101 at the exit of the nip. Such a melted toner image having a smooth surface (i.e., a high gloss) then gradually roughens its surface as the toner image separates from the heating roller 101, due to a force of separation acting against a force of adhesion exerted between the toner image and the heating roller 101 and increase of aggregation force of toner particles along with temperature decrease. Consequently, the toner image having a roughened surface is cooled and fixed on the recording material, providing a lower gloss. Conventional fixing devices, including both roller-type and belt-type fixing devices, have such a problem of decrease in image gloss.
In view of this situation, various attempts have been made to increase gloss of a toner image fixed on a recording material.
For example, Japanese Patent Application Publication No. S63-192068 (JP-S63-192068-A) proposes a method of increasing image gloss by passing a toner image through a nip formed between a heating roller and a pressing roller twice or more.
JP-H01-256283-A proposes a method of increasing image gloss by providing two nips, the first nip and the second nip. A toner image can be selectively passed through either of the nips to control the resultant image gloss.
JP-S63-122666-A proposes a method of increasing image gloss by applying heat and pressure to a toner image while covering the toner image with a sheet having a smooth surface and separating the sheet from the recording material after cooling the toner image.
JP-H01-279277-A and JP-S63-6584-A each propose a method of increasing image gloss by laminating a toner image between a recording material and a transparent sheet.
JP-H02-162383-A proposes a method of increasing image gloss by contacting a fixing belt with a toner image on a recording material while heating the toner image by a heater provided on the opposite side of the fixing belt from its recording material contacting side, cooling the toner image while keeping the fixing belt contacting the toner image, and then separating the fixing belt from the toner image.
In a case in which a toner image is fixed on coated paper, generally having a relatively high gloss, the toner image preferably expresses a relatively high gloss similar to that of the coated paper, so that a natural image having uniform gloss throughout the image is produced. However, where a toner image expresses such a relatively high gloss even when fixing on a recording material having a relatively low gloss, the resulting image may be unnatural due to the disparity in glossiness between different parts of the image. For the above reasons, it is not always preferable that toner images have high gloss. In some cases, it is preferable that the toner images have low gloss, depending on the type of recording material in use and the type of toner image desired.
However, none of the above-described five related art references selectively control the resulting image gloss as described above.
Further, JP-2004-325934-A and JP-2004-167757-A each discloses a method of increasing image gloss without any additional material, such as a sheet or a laminate film. Specifically, the resulting image gloss is increased by contacting an endless belt with a melted toner image on a recording material while cooling the toner image, and separating the endless belt from the toner image. This method has a problem in that the toner image is likely to solidify while contacting the endless belt. The solidified toner image is difficult to separate from the endless belt, probably causing paper jam. Additionally, the disclosed mechanism of controlling the resulting image gloss by adjusting the length of contact between the endless belt and the recording material by changing the positions of multiple rollers stretching the endless belt has the added disadvantage of making the fixing device bigger.
FIG. 2 schematically illustrates a related-art fixing device having a configuration as described above.
The fixing device illustrated in FIG. 2 has a mechanism for increasing image gloss. Specifically, the fixing device includes a fixing unit 10 that fixes a toner image on a recording material by application of heat and pressure, and a separate glossing unit 20 located downstream from the fixing unit 10 that increases gloss of the toner image fixed on the recording material.
The fixing unit 10 has a similar configuration as the related-art fixing device illustrated in FIG. 1. The fixing unit 10 includes a heating roller 11 and a pressing roller 12. The heating roller 11 and the pressing roller 12 form a fixing nip therebetween while rotating.
The heating roller 11 is driven to rotate clockwise in FIG. 2 by a driving mechanism, not shown. The pressing roller 12 is pressed against the heating roller 11 and is rotated counterclockwise in FIG. 2 along with rotation of the heating roller 11, while forming a fixing nip with the heating roller 11.
The heating roller 11 is constituted as a cylindrical metal core covered with a heat-resistant elastic layer comprised of a silicone rubber, etc. The heating roller 11 incorporates a halogen heater 13 that applies heat to toner images. Similarly, the pressing roller 12 is constituted as a cylindrical metal core covered with a heat-resistant elastic layer comprised of a silicone rubber, etc., incorporating a halogen heater 14.
Thermistors 15 and 16 are provided contacting the heating roller 11 and the pressing roller 12, respectively. The thermistors 15 and 16 detect the surface temperatures of the heating roller 11 and the pressing roller 12, respectively. A controller, not shown, turns the halogen heaters 13 and 14 on and off so that the heating roller 11 and the pressing roller 12 maintain predetermined temperatures.
The glossing unit 20 includes an endless belt 24 stretched taut with a heating roller 21, a supporting roller 26, and a separating roller 27. A pressing roller 22 is pressed against the heating roller 21 with the endless belt 24 therebetween. The heating roller 21 incorporates a halogen heater 23. A thermistor 25 is provided contacting the heating roller 21 to detect the surface temperature of the heating roller 21. A controller, not shown, turns the halogen heater 23 on/off so that the heating roller 21 maintains a predetermined temperature.
A cooler 28 is provided so as to face an inner circumferential surface of the endless belt 24. The cooler 28 cools the endless belt 24 and a recording material intimately contacting the endless belt 24, immediately downstream from where the pressing roller 22 is pressed against the heating roller 21. The cooler 28 may be a fan mechanism that cools the endless belt 24 by blowing air or a Peltier cooling mechanism that cools the endless belt 24 with a Peltier element that is a semiconductive element using the Peltier effect.
Operations of the fixing device illustrated in FIG. 2 are described below.
In the fixing unit 10, a recording material having an unfixed toner image thereon passes through the fixing nip formed between the heating roller 11 and the pressing roller 12, while the halogen heater 13 heats the heating roller 11 to a predetermined temperature and the pressing roller 12 presses the heating roller 11 with a predetermined pressure. Thus, the toner image is completely fixed on the recording material after passing through the fixing unit 10.
In the downstream glossing unit 20, the recording material passed through the fixing unit 10 then passes through the nip formed between the heating roller 21 and the pressing roller 22, so as to increase image gloss of the fixed toner image. Since the toner image is already completely fixed on the recording material in the fixing unit 10, the heating roller 21 applies only enough heat to level the surface of the toner image. For example, the heating roller 21 may be heated to a temperature between the temperature of the recording material at entry into the glossing unit 20 and that at immediately after discharge from the fixing unit 10. Alternatively, the heating roller 21 may be heated to a temperature between the softening temperature and the ½ flow starting temperature of the toner in use, for example, between 60° C. and 120° C. Thus, only the surface of the toner image, not the entire toner image, is softened and smoothened by contact with the endless belt 24 having a smooth surface, without degrading color tone of the entire toner image. The toner image thus given a high gloss is then conveyed a predetermined distance, and separated from the endless belt 24 by the curvature of the separating roller 27.
On the other hand, the cooler 28 cools the recording material as well as the endless belt 24, while the recording material is intimately contacting the endless belt 24. Therefore, the recording material has a lower temperature when separating from the endless belt 24 by the separation roller 27 than when passing through the nip formed between the heating roller 21 and the pressing roller 22. This means that the cooled and solidified toner image separates from the endless belt 24, which can maintain a high gloss of the toner image. It is to be noted that the cooler 28 is not necessary when the recording material can be cooled simply by intimate contact with the endless belt 24.
Generally, when gloss of a toner image is substantially the same as that of a recoding material on which the toner image is formed, we sense that the image is natural. There is a wide variety of recording materials, from low-gloss recording materials to high-gloss recording materials. For example, coated paper, widely used for brochures, has a glossiness of about 45%. When a toner image is formed on such a high-gloss coated paper, the toner image is required to express the same glossiness of about 45%. As another example, photographic images generally require a high glossiness of about 70 to 90%. In this specification, the glossiness (%) is measured with a gloss meter, the angle of incidence of which is set to 60°.
Gloss of a toner image varies depending on the temperature of the heating roller 11. The higher the temperature of the heating roller 11, the higher the gloss of the toner image. However, when the heating roller 11 is excessively heated with intent to increase the gloss of the toner image, it is likely that hot offset problem disadvantageously occurs. This is the reason why the glossing unit 20 is preferably provided immediately downstream from the fixing unit 10, rather than merely increasing the temperature of the heating roller 11.
The toner image has a glossiness of from 20 to 25% after passing through the glossing unit 20, even when the toner image is formed on a low-gloss paper having a glossiness of 5 to 10%. Such an inconsistency in glossiness between the toner image and the paper provides us with a sense of discomfort. The below-described fixing device solves such a problem.
FIG. 3 schematically illustrates another related-art fixing device which can control image gloss depending on the type of paper in use. The fixing device illustrated in FIG. 3 has the same configuration as that illustrated in FIG. 2 except that the position of the separating roller 27 is variable.
More specifically, the separating roller 27 is horizontally movable toward the heating roller 21 while keeping the recording material intimately contacting a lower surface of the endless belt 24, stretching between the heating roller 21 and the separating roller 27. The length of contact, in other words, the time of contact, between the recording material and the endless belt 24 is controllable by controlling the extent of movement of the separating roller 27. The supporting roller 26 is also movable along with movement of the separating roller 27 to keep the endless belt 24 stretched taut.
The fixing device illustrated in FIG. 3 operates in the same manner as that illustrated in FIG. 2 except that the separation roller 27 moves toward the heating roller 21 depending on the gloss of recording material in use, so as to control the time and/or length of contact between the recording material and the endless belt 24. Thus, the resulting image gloss is controllable. For example, when forming a photographic image on a high-gloss coated paper, the separating roller 27 is preferably positioned as indicated by dotted lines in FIG. 3, so that the time and/or length of contact between the recording material and the endless belt 24 becomes maximal. By contrast, when forming a much lower-gloss toner image, the separating roller 27 is preferably positioned as indicated by solid lines in FIG. 3, so that the time and/or length of contact between the recording material and the endless belt 24 are shortened. In this case, at the moment of separation of the toner image from the endless belt 24, the toner image has not been cooled as much as in a case in which the separating roller 27 is positioned as indicated by dotted lines in FIG. 3. Therefore, the toner image is likely show reduced gloss when separating from the endless belt 24.
However, the fixing device illustrated in FIG. 3 has a disadvantage in that horizontal movement of the separating roller 27 is limited due to the presence of the cooler 28, and therefore the time and/or length of contact between the recording material and the endless belt 24 are also limited. Such a fixing device does not satisfactorily meet the demand for controlling image gloss in accordance with divers recording materials. Additionally, providing space for moving the supporting roller 26 along with movement of the separating roller 27 hinders efforts to meet market demand for downsizing the fixing device.